The invention generally relates to spectral enhancement systems for enhancing a spectrum of multi-frequency signals (e.g., acoustic, electromagnetic, etc.), and relates in particular to spectral enhancement systems that involve filtering and amplification.
Conventional spectral enhancement systems typically involve filtering a complex multi-frequency signal to remove signals of undesired frequency bands, and then amplifying the filtered signal in an effort to obtain a spectrally enhanced signal that is relatively background free.
In many systems, however, the background information may be difficult to filter out based on frequencies alone because the complex multi-frequency signal may include background noise that is close to the frequencies of the desired information signal. Moreover, many conventional spectral enhancement systems inadvertently amplify some background noise with the amplification of the desired information signal.
For example, a spectral enhancement system may include one or more band pass filters into which an input signal is received, as well as one or more compression and/or amplification units, the outputs of which are combined at a combiner to produce an output signal. If the frequencies of the desired signals, for example, vowel sounds in an auditory signal are either within a band filtered frequency or are surrounded by substantial noise signals in the frequency spectrum, then such a filter and amplification system may not be sufficient in certain applications.
As a particular example of a spectral enhancement system, an electronic cochlea models the traveling-wave amplifier architecture of a biological cochlea as a cascade of nonlinear-and-adaptive second-order filters with corner frequencies that decrease exponentially from approximately 10 kHz to 100 Hz. Due to the successive compounding of gains, a change in the individual filter gains of a few percent can alter the gain of the composite transfer function by many orders of magnitude. For example, (1.1)45=73 while (0.9)45=0.009. It is very difficult to accomplish such wide-dynamic-range gain control with one localized amplifier without changing the amplifier's bandwidth, temporal resolution, and power dissipation drastically. Any parameter variations in the Q's of the various cochlear filters, which can result in inhomogenities and nonrobust or unstable operation, are compensated for through gain control. Any physical biological system, such as the cochlea, must possess a feedback system to ensure that it works in a real-world environment, where parameters are not perfectly matched and controlled to high precision as in current digital implementations or simulations.
Distributed gain control and the traveling-wave phenomena are important aspects of the silicon cochlea (as disclosed in A Low-Power Wide-Dynamic Range Analog VLSI Cochlea, Sarpeshkar, R., Lyon, R. F., and Mead, C. A., Analog Integrated Circuits and Signal Processing (1998), the disclosure of which is hereby incorporated by reference) in replicating the performance of the biological cochlea. The silicon cochlea's importance to cochlear implant processing is significant for at least the following reasons.
1) An exponentially tapering filter-cascade architecture provides an extremely efficient mechanism for constructing a bank of closely spaced high-order filters as disclosed in Traveling Waves versus Bandpass Filters: The Silicon and Biological Cochlea, Sarpeshkar, R., Proceedings of the International Symposium on Recent Developments in Auditory Mechanics, World Scientific (2000), and Filter Cascades as Analogs of the Cochlea, Lyon, R. F., Neuromorphic Systems Engineering (1998), the disclosures of which are both hereby incorporated by reference. As the number of channels in implants continues to grow (e.g., 31 channel implants, 64-channel implants, 128-channel implants etc.), the advantages of filter cascades in creating a bank of high-order filters will become more and more apparent.
2) A sophisticated frequency-dependent version of the gain control algorithms presently used in implants and hearing aids may be implemented as disclosed in Comparison of Different Forms of Compression in Wearable Digital Hearing Aids, Stone, M. A., Moore, B. C. J., Alcantara, J. I., and Glasberg, B. R., J. Acoustic Society of America, (1999), the disclosure of which is hereby incorporated by reference. Thus loud sounds at one frequency do not have to result in inaudible sounds at another frequency. Also, the gain control allows important phenomena in the perception of speech in noise such as forward masking to be easily modeled. Gain control has been shown to be particularly important in the performance of speech recognition systems in reverberant and noisy environments.
3) The architecture of the cochlea is amenable to both time and place coding as described in A Low-Power Analog Front-end Module for Cochlear Implants, Wang, R. J. W, Sarpeshkar, R, Jabri, M. and Mead, C. XVI World Congress on Otorhinolaryngology (1997), the disclosure of which is hereby incorporated by reference.
4) The biological realism allows several important phenomena in biology to be naturally replicated. These include filter broadening with level, the distributed coding of loudness, the transition from place cues to time cues as level increases, redundant signal representations, the close intertwining of both filtering and compression rather than the artificial separation of filtering and compression in today's implants, compression of long-term information while preserving good sensitivity to transients, two-tone suppression, the upward spread of masking, and forward masking. Although it is quite possible that none of these effects have any importance for implant patients, given that cochlear front ends have been shown to improve speech recognition in noise it is unlikely that models closer to the biology will have no impact on implant patients. It is also likely that coding strategies that are closer to the biology will prove superior to those that are not.
5) The silicon cochlea's analog circuit techniques provide a foundation for ultra-low-power cochlear implant design.
The silicon cochlea may be implemented as a particular form of local feedforward gain control as disclosed in A Low-Power Wide-Dynamic Range Analog VLSI Cochlea discussed above. Such an implementation, however, generates input-output curves that are too compressive as compared with those in a real cochlea. Such curves are not suitable for direct use in cochlear implants. Furthermore, such curves cannot easily be programmed to implement a desired compression characteristic, an important necessity in a practical system.
There is a need therefore, for an improved spectral enhancement system that is efficient and practical.